A pilot signal is commonly used for communication systems to enable a receiver to perform a number of critical functions, including but not limited to, the acquisition and tracking of timing and frequency synchronization, the estimation and tracking of desired channels for subsequent demodulation and decoding of the information data, the estimation and monitoring of the characteristics of other channels for handoff, interference suppression, etc. Several pilot schemes can be utilized by communication systems, and typically comprise the transmission of a known sequence at known time intervals. A receiver, knowing the sequence only or knowing the sequence and time interval in advance, utilizes this information to perform the abovementioned functions.
For the uplink of future broadband systems, single-carrier based approaches with orthogonal frequency division are of interest. These approaches, particularly Interleaved Frequency Division Multiple Access (IFDMA) and its frequency-domain related variant known as DFT-Spread-OFDM (DFT-SOFDM), are attractive because of their low peak-to-average power ratio (PAPR), frequency domain orthogonality between users, and low-complexity frequency domain equalization.
In order to retain the low PAPR property of IFDMA/DFT-SOFDM, only a single IFDMA code should be transmitted by each user. This leads to restrictions on the pilot symbol format. In particular, a time division multiplexed (TDM) pilot block should be used, where data and pilots of a particular user are not mixed within the same IFDMA block. This allows the low PAPR property to be preserved and also enables the pilot to remain orthogonal from the data in multi-path channels, since there is conventionally a cyclic prefix between blocks. An example is shown in FIG. 1, where an IFDMA pilot block and subsequent IFDMA data blocks for a transmission frame or burst are shown.
While the TDM pilot approach is attractive for PAPR and pilot orthogonality, it limits the granularity available for specifying the pilot overhead, especially when a transmission frame or burst has a small number of IFDMA blocks. A second issue is that the pilot configuration remains fixed and is not adapted to the changing channel conditions. Therefore a need exists for a method and apparatus for pilot signal transmission that alleviates the above-mentioned problems.